The present invention relates to a method and apparatus for guiding the drilling of bore holes with respect to a target such as a blowout well, and more particularly to a system for determining the distance from a directional borehole to a target well bore by the measurement of electric fields.
The use of magnetic field measurements for determining the location of a cased target well bore is described in U.S. Pat. No. 4,372,398 which issued on Feb. 8, 1983 to the applicant herein, which patent is hereby incorporated herein by reference, in its entirety. In accordance with that patent, a low frequency alternating electric current flow is produced in the casing of the target well bore. During periodic interruptions in drilling of a directional, or relief, borehole, measurements are taken at selected depth intervals of the magnitude and direction of the magnetic field which is produced in the relief borehole by the current flow in the target well casing. At the same time, measurements are made of the magnitude and direction of the earth's magnetic field so that the orientation of the measurement device can be determined. From these measurements, the compass direction to the target well bore can be determined. By plotting changes in the amplitude and direction of the source of the magnetic field over the course of a number of measurements, and knowing the resistivity of the target well casing and thus the magnitude of the current flowing therein, estimates of the distance to the target well by triangulation can be made with good accuracy. A suitable magnetometer for use in making such magnetic field measurements is illustrated in U.S. Pat. No. 4,323,848, also issued to the present applicant, which patent is incorporated herein by reference, in its entirety.
U.S. Pat. No. 4,700,142, also issued to the applicant herein, discloses a method for locating a target well by means of a magnetic field produced by current flowing in the casing of such a well, but without the need to use the earth's magnetic field for orientation purposes. In this case, the orientation of the measuring device is fixed by various combinations of gyroscopes, accelerometers, magnetometer measurements, and bore hole survey data.
U.S. Pat. No. 4,933,640, issued Jun. 12, 1990 to the applicant herein also discloses a method and apparatus for determining the distance and direction of a target well from a directional borehole being drilled. In this case, a toroidal inductor surrounds a drill stem within the borehole being drilled, and this inductor acts as the source of ah AC current which is caused to flow in the target well bore to produce a secondary magnetic field which can be detected.
The determination of distance and direction by the measurement of magnetic fields in the manner disclosed in the foregoing patents has worked well for a number of years in guiding the drilling of directional wells to intersect (or to avoid) target well casings. However, there has been a long felt need for a more effective way to determine distance to a target in those circumstances when it is not possible to use magnetic field triangulation techniques. Such techniques require fitting the observed changes in the measurements of direction to the target versus depth, which information is obtained by means of well surveys conducted in accordance with the foregoing patents. A mathematical evaluation of the electromagnetic coupling between the current flowing in the target well and the excitation system is not required in this case. However, triangulation methods are not satisfactory when the relief well is on a "head on" drilling course directly toward a target well, and the determination of distance then involves intricate three-dimensional computations using the electrical resistivity of the earth in the region of the target well, the amount of steel or other electrically conductive material per foot in the casing or drill stem contained within the target well bore, as well as detailed survey information of the paths of both the target and the relief boreholes. Although such techniques have been reasonably successful, on occasion situations occur where the methods of computation available utilizing such magnetic field measurements also are unsuitable for carrying out accurate distance evaluations. For example, there are times when the weight of the steel in the target well is not known, or survey information is not available, or the electrical resistivity of the earth surrounding the target well and between the target and relief well is not known. Thus, there has arisen a need for an alternate method for determining the distance between a directional, or relief, borehole being drilled and a target well bore which is to be intersected (or avoided) by the directional borehole.